Runaway greenhouse gas losing its steam.

Although the international community missed its opportunity in Brazil last month to set limits on carbon dioxide emissions, not all the news under the greenhouse is bad. Without even trying, humans have apparently succeeded in slowing the atmospheric buildup of methane, another powerful greenhouse gas, scientists reported this week.

Measurements made around the world reveal that while concentrations of methane continue to increase, they are not rising as quickly now as they were almost a decade ago. In 1983, methane levels were climbing at 13.3 parts per billion per year. But by 1990, the rate of increase had dropped to 9.5 parts per billion per year, according to researchers with the National Oceanic and Atmospheric Administration (NOAA), the University of Colorado at Boulder, and the Australian Commonwealth Scientific and Industrial Research Organization, based in Mordialloc, Victoria. They describe their findings in the July 23 NATURE.

Climate experts worry about rising methane levels because the buildup of this gas accounts for roughly 15 to 20 percent of the greenhouse warming power added to the atmosphere each year. Methane comes from natural sources as well as from human activities such as cattle rearing, rice farming, and the mining of fossil fuels.

The researchers analyzed a data set of roughly 10,000 air samples, collected at 37 sites scattered around the world, mostly on islands. While the scientists cannot pinpoint what has put the brakes on the methane buildup, the pattern of recent changes offers some clues.

The data indicate that the greatest slowdown in methane accumulation has occurred in the higher latitudes of the northern hemisphere. "The fact that that's where most of the industrialized world lives, in that latitude zone, suggests that there's some human involvement in this decrease we're seeing," says Edward J. Dlugokencky a NOAA researcher based in Boulder.

Other groups have also detected a drop in the methane accumulation rate, but they relied on less extensive sampling networks that cannot reveal as much about the regional pattern of methane changes, Dlugokencky says. If the present trend continues, methane concentrations will level off in about 15 years. But without knowing what has caused the changes, scientists cannot predict what will actually happen with methane, Dlugokencky adds.

F Sherwood Rowland, an atmospheric chemist at the University of California, Irvine, mentions several factors that could explain why the buildup of methane has slowed. Between 1950 and 1975, the number of cattle in the world increased from 800 million to 1.25 billion, but since 1975 the cattle population has leveled out. Rice production seems to have followed a similar pattern of rapid increase followed by a slowdown. Lastly, oil companies apparently changed their practices during the late 1970s to cut down on methane loss during oil extraction. Because methane persists in the atmosphere for a decade, any or all of these changes could have caused a slowdown in methane buildup during the 1980s.